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Ballast Resistor

Ballast Resistor


A Jag-lover asks:

I know the British think of everything, but I'm almost certain the smoke coming from that resistor bolted to the side of the coil is not a airshow style smoke system.

At first I thought it was just some oil had splashed on it and would burn off. But, she's still smoking... Car seems to run fine, but this really can't be right. What purpose does that resistor serve anyway.

It's the ballast resistor.

Here's the scoop, it's a 6 volt coil! The resistor is there to drop the 12 volts (actually more like 13.8 volts) from the cars electical system down to 6-ish volts for the coil. Don't panic, almost all cars are set up this way. Here's why:

(Very generalized discription follows. Your mileage may vary!)

When you try to start the car, and while the starter motor is cranking the engine, the current drain on the battery is so high that the electrical system voltage drops (Every noticed how the lights sometimes flicker if you try to start the car at night with the lights on?) by as much as half. So we install a 6 volt coil in the hope that it will be able to produce enough of a spark to start the engine while it's beeing cranked. But, after the engine starts and the alternator goes to work recharging the battery, the system voltage comes back up to 13.8 volts. So, as soon as you release the key switch from the start position, that resistor is switched into the coil primary circuit to reduce the voltage to 6 volts, and the coil is happy.


"Ah," you say, "but my brother-in-law has a ( insert your favorite sub-compact here) and I've worked on it, and I know that it doesn't do that." says you, "I'm certain that there is only one skinny wire to from the coil to the distributor and one skinny wire to the keyswitch." says you, thinking that you can throw ole' Marv a curve here.

Sigh... All cars are build to a price spec. Some manufacturers eliminate that extra wire and switch contact set in the keyswitch in order to save a buck, but use the same 6 volt coil as everyone else 'cause they are cheaper 'cause they are mass-produced. Besides, a little tiny 4 cylinder engine in your typical rice- burner is pretty easy to crank, hence much less strain on the battery and such, so more voltage is available to fire the coil. All it has to do is manage to fire a cylinder or two and it's running.

"So, why does mine smoke?" says you. Well, it shouldn't, so there is a problem. The resister gets rid of that extra 6 volts by turning it into heat. Yours is making to much heat, so it's being asked to eat too much voltage. a couple of possibilities:

  1. The coil is partially shorted, so it's not using up it's fair share of the 13.8 volts available, and this forces the ballast resistor to over-eat, because it's gotta go somewhere, and it over-heats (this is my guess)
  2. The dwell is too long. These parts are made to work only part time. The points in the distributor close, then open (or this close-open function is accomplished electronically if it's a distributor without points.) If they are staying closed for too long (dwell too long) current flows through the coil (and resistor) for too long a time, and it's rest period is too short, so it overheats and smokes.
  3. The resistor is partially shorted, so this allows the current flow through it to be too high, so it overheats. This failure is uncommon becuse when the resistor is abused it will usually burn open insteat of partially short. Besides, this failure will ussually take out the coil before the resistor smokes.
  4. A combination of failures. The points stick, which causes the coil to overheat so it developes a partial short, which causes the resistor to get hot, so it developes a short etc. You get the idea.

Does any of this long-winded rambling help?

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